1. Field of the Invention
The present invention relates to a horizontal oscillator circuit for use in a CRT automatic scanning monitor. More particularly, the invention relates to a technique for making a stable change in the frequency of an output signal from a horizontal oscillator circuit when a change occurs in the frequency of an input horizontal synchronizing signal inputted to the horizontal oscillator circuit.
2. Description of the Background Art
At the present time, CRT display monitors are utilized in various applications. In particular, automatic scanning monitors which are capable of handling input video signals based on various standards by using only a single display monitor and of automatically discriminating between such various input video signal standards to display images conforming to the respective standards, have found widespread applications.
To handle input video signals having frequencies ranging from 15.75 kHz which is a television frequency to 100 kHz which is required for use as a workstation display monitor, such an automatic scanning monitor comprises a horizontal oscillator circuit capable of generating such a wide range of horizontal frequencies. The construction and basic operation of the horizontal oscillator circuit is described with reference to FIG. 7. FIG. 7 is a circuit diagram schematically showing the general construction of a horizontal deflection circuit 161 for a CRT display monitor.
As illustrated in FIG. 7, the horizontal deflection circuit 161 comprises a horizontal oscillator circuit 151 which includes a horizontal oscillation IC 101, two capacitors 108a and 108b, a transistor 109, and a diode 110.
The horizontal oscillation IC 101 has a terminal T1011 connected to an interconnect line through which an input horizontal synchronizing signal HD is transmitted. The interconnect line is branched at a midpoint for connection to a terminal T1122 of a microprocessor 112.
The horizontal oscillation IC 101 further includes a terminal T1012 connected commonly to respective first ends of the capacitors 108a and 108b. The capacitor 108a has a second end grounded. The capacitor 108b has a second end connected commonly to a cathode terminal of the diode 110 and a collector terminal of the transistor 109. An anode terminal of the diode 110 and an emitter terminal of the transistor 109 are grounded. The transistor 109 has a base terminal connected to a terminal T1121 of the microprocessor 112. The two capacitors 108a and 108b which control the frequency of an output signal VT1014 from the horizontal oscillator circuit 151 as will be described later are also referred to generically as an "oscillating capacitor 108."
A circuit 141 including the transistor 109 and the diode 110 functions as a switch for (electrically) connecting the capacitor 108b to the terminal 1012 or to the first end of the capacitor 108a or for disconnecting the capacitor 108b from the first end as will be described later during the operation of the horizontal oscillation IC 101. Thus, the circuit 141 is referred to hereinafter as a "switching circuit 141."
The horizontal oscillation IC 101 further includes a terminal T1013 connected commonly to a first end (positive terminal) of a capacitor 115 and a first end of a resistor 116. The capacitor 115 has a second end (negative terminal) grounded. The resistor 116 has a second end connected to a terminal T1071 of a D/A converter 107. The capacitor 115 and the resistor 116 function as a filter. A terminal T1073 of the D/A converter 107 and a terminal T1123 of the microprocessor 112 are connected to each other through a predetermined interconnect line.
The horizontal oscillation IC 101 further includes a terminal T1014 connected to an input terminal of a horizontal drive circuit 102. The horizontal drive circuit 102 has an output terminal connected to a first input terminal of a horizontal output circuit 103. The horizontal output circuit 103 has an output terminal connected to a terminal T1015 of the horizontal oscillation IC 101.
The horizontal output circuit 103 has a second input terminal connected through a choke coil 104 to an output terminal of a variable power supply circuit 105. The variable power supply circuit 105 has a first input terminal connected to an output terminal of a power supply circuit 106, and a second input terminal connected commonly to a first end (positive terminal) of a capacitor 117 and a first end of a resistor 118. The capacitor 117 has a second end (negative terminal) grounded, and the resistor 118 has a second end connected to a terminal T1072 of the D/A converter 107. The capacitor 117 and the resistor 118 function as a filter.
A terminal T1124 of the microprocessor 112 is connected to an input terminal of the power supply circuit 106.
The basic operation of the horizontal deflection circuit 161 of FIG. 7 is discussed below.
First described is the operation when the switching circuit 141 is OFF, that is, when the transistor 109 is OFF and the terminal T1012 of the horizontal oscillation IC 101 is connected only to the capacitor 108a.
The horizontal oscillator circuit 151 generates a voltage pulse (also referred to simply as a "pulse") VT1014 having the same frequency and the same phase as the input horizontal synchronizing signal HD applied to the terminal T1011 to output the voltage pulse VT1014 at the terminal T1014. The horizontal drive circuit 102 amplifies the pulse VT1014 to output a signal VT102. The horizontal output circuit 103 is driven by the signal VT102. At this time, an output signal VT1031 from the deflection circuit 161 is inputted (fed back) to the terminal T1015 of the horizontal oscillation IC 101. Thus, the horizontal deflection circuit 161 includes a loop circuit having a signal-flow path extending from the terminal T1014 through the horizontal drive circuit 102 and the horizontal output circuit 103 to the terminal T1015. The horizontal oscillation IC 101 compares the phase of the feedback signal VT1031 with the phase of the input horizontal synchronizing signal HD serving as a reference signal to generate the pulse VT1014 in phase with the input horizontal synchronizing signal HD. Thus, the loop circuit including the horizontal oscillation IC 101 functions as a PLL circuit for generating the signal VT1014 synchronized with the input horizontal synchronizing signal HD.
In particular, the free-running frequency of an oscillation portion included in the horizontal oscillation IC 101 is controlled by the charging and discharging of the capacitor 108a connected to the terminal T1012 (or the two capacitors 108a and 108b if they are connected to the terminal T1012). Since such charging and discharging are performed so that a voltage VT1012 at the terminal T1012 falls within a range from a voltage level Vmin to a voltage level Vmax as shown in FIG. 8, the voltage VT1012 has a sawtooth voltage waveform. A constant current source included in the horizontal oscillation IC 101 performs the above-mentioned charging and discharging, and provides a current value controlled by the voltage level at the terminal T1013.
Thus, the rate of change in the sawtooth voltage shown in FIG. 8 (the slope of the graph of FIG. 8) in accordance with the above charging and discharging operation is determined by the capacitance of the oscillating capacitor 108 connected to the terminal T1012 and the output current from the constant current source (the voltage applied to the terminal T1013 accordingly). In other words, the free-running frequency of the oscillation portion in the horizontal oscillation IC 101 is controllable by the capacitance of the oscillating capacitor 108 and the voltage level VT1071 at the terminal T1013. As shown in FIG. 7, a predetermined signal from the microprocessor 112 is applied as the output voltage VT1071 from the D/A converter 107 to the terminal T1013. Finally, the loop circuit (PLL circuit) including the horizontal oscillation IC 101 generates and outputs the output pulse VT1014 having the same frequency and the same phase as the input horizontal synchronizing signal HD.
A predetermined power supply voltage V105 is applied from the variable power supply circuit 105 through the choke coil 104 to the horizontal output circuit 103. A power supply voltage V106 from the power supply circuit 106 is applied to the variable power supply circuit 105. At this time, a predetermined signal from the microprocessor 112 is inputted as an output voltage VT1072 from the D/A converter 107 to the variable power supply circuit 105. The variable power supply circuit 105 generates the voltage V105 based on the voltage VT1072 to output the voltage V105 to the horizontal output circuit 103.
The above description may be applied to the operation in a steady state wherein the capacitor 108b is connected to the terminal T1012 of the horizontal oscillation IC 101 and the oscillating capacitor 108 is comprised of the two capacitors 108a and 108b.
In the horizontal deflection circuit 161, the microprocessor 112 changes an output voltage VT1121 at the terminal T1121 to control the ON/OFF states of the transistor 109, changing the capacitance of the oscillating capacitor 108. Specifically, the microprocessor 112 turns OFF the transistor 109 to connect only the capacitor 108a to the terminal T1012 of the horizontal oscillation IC 101 (state (i)), whereas the microprocessor 112 turns ON the transistor 109 to connect the two capacitors 108a and 108b to the terminal T1012 (state (ii)). Changing between the two states (i) and (ii) is referred to hereinafter as "switching the oscillating capacitor 108."
In each of the states (i) and (ii), the horizontal oscillation IC 101 can generate a pulse having a frequency within a range of about (maximum frequency)/(minimum frequency).ltoreq.3 by controlling the voltage level applied at the terminal T1013. For example, in the state (i), the horizontal deflection circuit 161 designed to handle the input horizontal synchronizing signal HD having a frequency range from 45 kHz to 100 kHz can accommodate a change in the frequency of the input horizontal synchronizing signal HD between 48 kHz and 64 kHz, with the state (i) remaining intact. In the state (ii), the horizontal deflection circuit 161 designed to handle the input horizontal synchronizing signal HD having a frequency range from 15.75 kHz to 45 kHz can accommodate a change in the frequency of the input horizontal synchronizing signal HD between 15.75 kHz and 31.5 kHz, with the state (ii) remaining intact.
On the other hand, when the frequency of the input horizontal synchronizing signal HD changes, for example, from 64 kHz to 15.75 kHz, the horizontal deflection circuit 161 must accommodate the change by changing the transistor 109 from the OFF state to the ON state. In other words, when such a frequency change is made, the horizontal deflection circuit 161 switches the oscillating capacitor 108 from the state (i) in which only the capacitor 108a is connected to the terminal T1012 to the state (ii) in which the two capacitors 108a and 108b are connected to the terminal T1012.
Conversely, when the frequency of the input horizontal synchronizing signal HD changes, for example, from 15.75 kHz to 64 kHz, the horizontal deflection circuit 161 changes the transistor 109 from the ON state to the OFF state to switch the oscillating capacitor 108 from the state (ii) to the state (i).
Description is given on the operation of the horizontal deflection circuit 161 or the horizontal oscillator circuit 151 when the oscillating capacitor 108 is switched from the state (i) in which only the capacitor 108a is used as the oscillating capacitor 108 to the state (ii) in which the two capacitors 108a and 108b are used, with reference to FIG. 7.
The microprocessor 112 always receives the input horizontal synchronizing signal HD to measure the frequency of the input horizontal synchronizing signal HD. Upon detecting a change in the frequency of the input horizontal synchronizing signal HD, the microprocessor 112 transmits to the power supply circuit 106 a signal VT1124 for causing the power supply circuit 106 to suspend the supply of power to the variable power supply circuit 105.
At the time that the power supply voltage V105 supplied from the variable power supply circuit 105 to the horizontal output circuit 103 reaches approximately 0 V after the power supply circuit 106 suspends the supply of power to the variable power supply circuit 105 based on the signal VT1124, the microprocessor 112 outputs (applies) the voltage VT1121 for turning ON the transistor 109. At this time, the microprocessor 112 outputs the voltage VT1071 and the voltage VT1072 corresponding to the new frequency of the input horizontal synchronizing signal HD through the D/A converter 107 to the horizontal oscillation IC 101 and the variable power supply circuit 105, respectively.
After a lapse of predetermined time (previously set) required until the frequency of the output pulse VT1014 from the terminal T1014 of the horizontal oscillation IC 101 is stabilized, the microprocessor 112 transmits to the power supply circuit 106 the signal VT1124 for causing the power supply circuit 106 to restart the supply of power to the variable power supply circuit 105.
Through the above described operation, the horizontal oscillator circuit 151 generates the output pulse VT1014 in accordance with the new input horizontal synchronizing signal HD, and then the horizontal deflection circuit 161 outputs the signal synchronized with the new input horizontal synchronizing signal HD based on the pulse VT1014.
In the horizontal deflection circuit 161, as above described, the variable power supply circuit 105 is temporarily powered OFF for the connection of the capacitor 108b to the terminal T1012. The necessity for the power-off operation is described below.
The frequency of the output pulse VT1014 from the horizontal oscillation IC 101 changes abruptly at the instant that the capacitor 108b is connected to the terminal T1012. The output voltage V105 from the variable power supply circuit 105 which is newly set in accordance with the change in the frequency of the input horizontal synchronizing signal HD is incapable of following the abrupt frequency change of the voltage VT1014 because of the presence of a circuit component such as the choke coil 104. An attempt to abruptly change the output voltage V105 from the variable power supply circuit 105 so as to follow the frequency change results in an overshoot in the output voltage V105. Thus, electrical stresses are applied to the horizontal output circuit 103 if the frequency of the pulse VT1014 abruptly changes while the variable power supply circuit 105 continues supplying power to the horizontal output circuit 103. Under such circumstances, a mismatch (incompatibility) occurs, in the horizontal output circuit 103, between the frequency of the input pulse VT1014 from the horizontal oscillation IC 101 (or the input pulse VT102 from the horizontal drive circuit 102) and the voltage level supplied from the variable power supply circuit 105. This results in the electrical stresses applied to components constituting the horizontal output circuit 103. Such stresses might cause trouble in the horizontal output circuit 103 and the like. To avoid the electrical stresses and yet the trouble, the horizontal deflection circuit 161 is adapted to temporarily power OFF the variable power supply circuit 105 as above described.
Similar events can occur when the capacitor 108b connected to the terminal T1012 (in the state (ii)) is disconnected therefrom (into the state (i)). Therefore, the horizontal deflection circuit 161 is also adapted to temporarily power OFF the variable power supply circuit 105 when the oscillating capacitor 108 is switched from the state (ii) to the state (i).
If there is no need to switch the oscillating capacitor 108, e.g. for frequency changes between 15.75 kHz and 31.5 kHz and between 48 kHz and 64 kHz, the capacitor 115 and the resistor 116 connected between the horizontal oscillation IC 101 and the D/A converter 107 or the capacitor 117 and the resistor 118 connected between the D/A converter 107 and the variable power supply circuit 105 can adjust the rate of change in the frequency of the output pulse VT1014 from the horizontal oscillation IC 101 or the rate of change in the output voltage V105 from the variable power supply circuit 105. In such cases, the above-mentioned electrical stresses in the horizontal output circuit 103 are suppressed, and hence there is no need to power OFF the variable power supply circuit 105.
Using the horizontal deflection circuit 161 which can accomplish the above operation, the automatic scanning monitor receives the video signals based on various standards which differ in the input horizontal synchronizing signal HD to correctly display images.
As described hereinabove, the background art horizontal deflection circuit 161 or the background art horizontal oscillator circuit 151 is adapted such that when there is a need to connect the capacitor 108b to the terminal T1012 of the horizontal oscillation IC 101 or disconnect the capacitor 108b therefrom, power supply from the power supply circuit 106 to the variable power supply circuit 105 is temporarily cut off and then restarted after a lapse of fixed time. Thus, the background art horizontal deflection circuit 161 has a drawback in that, when a change in the frequency of the input horizontal synchronizing signal HD occurs which involves the switching of the oscillating capacitor 108, the fixed time is required to provide a stable image after the frequency change (or the response to the frequency change of the input horizontal synchronizing signal HD is slow).